Crude petroleum normally contains mineral salts that may corrode refinery units; the salt is removed from the crude oil by a process known as “desalting”, in which hot crude oil is mixed with water and a suitable demulsifying agent to form a water-in-oil emulsion which provides intimate contact between the oil and water, transferring salt into the water. The salty emulsion is then passed into a high voltage electric field inside a closed separator vessel. The electric field forces water droplets to coalesce, forming larger water droplets. As the water droplet volumes increase, they settle to the bottom of the tank under gravitation. The desalted oil forms at the upper layer in the desalter from where it is continuously drawn off for distillation. The salty water is withdrawn from the bottom of the desalter.
During operation of desalter units, a stable emulsion phase (also known as a “rag layer”) of variable composition and thickness forms above the interface between the oil and the separated bulk water phase at the bottom of the desalter. This interface will be referred to here as “oil/bulk-resolved water interface”. The formation of a rag layer is mostly due to stability of the oil/bulk-resolved-water interface caused by natural surfactants (e.g. asphaltenes, naphthenic acid) and/or solids. Solids, particularly, can reside at the interface generating a physical barrier against the immersion of water droplets into the bulk water phase at the bottom of the desalter. The formation of the rag layer is especially problematic for crude with high amounts of natural surfactants and/or solids. The growth of the rag layer reduces workable volume and may cause shorting within the electric circuit and force unplanned and costly desalter shut down. Additionally, processing crudes with high rag layer formation tendencies in current desalter configurations may cause poor desalting (salt removal) efficiency as a result of the accumulation of solids at the bottom of the vessel, and/or a solids-stabilized rag layer leading to erratic level control and insufficient residence time for proper water/oil separation. Formation of the rag layers has become a major desalter operating concern, generating desalter upsets, increased preheat train fouling, and deteriorating quality of the brine effluent and disruption of the operation of the downstream wastewater treatment facilities.
The water content of the rag layer may range from 20 to 95% water with the balance being hydrocarbon (normally full range crude oil) and up to 5 weight percent inorganic solids. Precipitated asphaltenes, waxes, and paraffins may also be found at elevated levels in the rag layer (compared to the incoming crude oil) which combine with particulates (solids), to bind the mixture together to form a complex structure that is highly stable. Intractable emulsions of this kind comprising of oil, water and solids make adequate separation and oil recovery difficult. Often, these stable emulsions arising from the desalter are periodically discarded as slop streams. This results in expensive treating or handling procedures or pollution problems as well as the fact that crude oil is also lost with these emulsions and slop streams.
Refinery sites which process high solids-content crudes have the most pervasive problems with emulsion formation. Heavy crude oils and bitumens from Western Canada, which contain elevated levels of small clay fines and other small solids, are particularly prone to forming large volumes of highly stable emulsions. With such feeds, growth of the rag layer is more prevalent. These feeds are, however, being introduced to refineries in greater quantities despite two main disadvantages related to the efficacy of desalting. First, the viscosity of these crudes can be quite high, so transport of water through the feed is slower than in high API gravity crude. Second, the density mismatch between water and oil is lower, so the gravitational energy gradient is reduced compared to higher API gravity crudes. Growth of the rag layer in the desalter requires either the amount of crude passed through the desalter to be reduced or removal of the rag layer from the desalting vessel for external treatment.
Attempts to mitigate the effects of rag layer formation are normally carried out by withdrawal of the emulsion from the unit or by the addition of chemical demulsifiers upstream of a desalter. The use of demulsifiers has proven to be effective in reducing emulsion stability between electrodes in a desalter, but may not be effective in reducing the rag layer build-up which is mainly due to stability of the oil/bulk-resolved-water interface. The common practice for application of demulsifiers has been to add the chemical demulsifiers to the water, oil, or the emulsion before introducing the oil/water mixture to the electric field, as shown by the following references:
U.S. Pat. No. 5,746,908 (Mitchell/Phillips Petroleum), discloses the use of steam to form an emulsion and then adding demulsifier to the mixture.
U.S. Pat. No. 7,867,382 (Droughton) discloses the use of demulsifier and mesoporous materials for reducing water-in-oil emulsion stability.
U.S. Pat. No. 7,923,418 (Becker/Baker Hughes) discloses the use of acrylate polymer emulsion breakers for reducing stability of a water-in-oil emulsion.
U.S. Pat. No. 7,981,979 (Flatt/Nalco) discloses the use of siloxane cross-linked demulsifiers for reducing water-in-oil emulsion stability.
A shortcoming of the current practice is due, in part, to the inability of chemical demulsifiers to reach high enough concentrations at the oil/bulk-resolved-water interface, particularly at the beginning of the desalter operation. Accordingly, the need persists for more effective techniques for mitigating the effects of rag layer formation.